The present application relates to systems and processes for preparing a dispersion of solid particles having increased purity and reduced particle size.
Wet grinding takes place primarily in agitated grinding media mills. Agitated grinding media mills move grinding media relative to each other by a fast rotating agitator in a liquid phase. The particles are comminuted primarily between the colliding grinding media. Traditionally, wet grinding is used with high-energy agitated grinding media mills for different applications to generate micrometer-sized particles and nanoparticles.
The mechanical reduction of a product into micrometer-sized particles is determined by the product's mechanical material properties. The particle strength and the mass-specific comminution energy of a product increase strongly with decreasing particle size.
The contamination of the product by wear of the grinding media and/or the grinding chamber is common. (See CLEMENT, M., 1986. Verschleiss bei der Zerkleinerung. In: Uetz, H. (ed.) Abrasion and Erosion. München and Wien, Carl Hanser Verlag, pp. 468-533.). In general, the grinding media produce most of the wear and primarily consist of different materials in comparison to the product or material to be ground. Usually metallic, silicate or ceramic materials, such as chrome steel, glass, alumina or zirconium oxide, are used as grinding media. The contaminations by wear of these materials impair products that have high purity requirements, such as for example pharmaceuticals, food, inks and paints.
Some efforts have been made to minimize contamination of the material to be ground. For example, some have attempted to use small plastic grinding media to decrease wear of grinding media and mill parts to reduce the contaminations. E. Merisko-Liversidge and co-workers employed such techniques to successfully reduce the contaminations from 1000 ppm by mill parts made from chrome steel to 10 ppm by grinding media of plastic. (MERISKO-LIVERSIDGE, E.; LIVERSIDGE, G. G.; COOPER, E. R. 2003. Nanosizing: a formulation approach for poorly-water-soluble compounds. European Journal of Pharmaceutical Sciences 18, pp. 113-120.).
Of course, another possible technique for decreasing the amount of contaminations would be to decrease the processing time of substances ground in the milling chamber. Merely decreasing the processing time, however, will generally cause an increase in the resulting particle size.